Certain magnetic blood pumps include an impeller having a permanent magnetization. For example, the impeller may interact with electrical coils in a stator so that the impeller and stator cooperatively form an electric motor which drives the impeller when alternating currents are applied to the coils in sequence. Implantable blood pump performance is often monitored by a controller connected to the pump. Controllers for monitoring pumps can use control algorithms which determine a parameter based on a magnetic interaction between the impeller and the stator, and use that parameter to monitor the operation of the pump. For example, the interaction between the impeller and coils of the stator, which are momentarily inactive, generates a voltage referred to as “back electromagnetic force” or “back EMF”. As discussed in United States Patent Application Publication 2012/0245681, the magnitude of the back EMF in certain axial-flow blood pumps is related to axial force on the impeller, and can be used to estimate blood flow through the pump.
However, when nominally identical pumps are made in serial production, there is considerable variability in the strength of the magnetic interaction between the impeller and stator in different pumps even through the pumps are nominally of identical construction. For example, different pumps in the series will yield different back EMF under identical operating conditions. This arises due to differences in the magnetic strength of the impellers, differences in the properties of the stators, or both. In particular, the magnetic strength of the impellers may vary due to subtle differences in microstructure of the metals constituting the impeller. It is difficult to eliminate these differences in production. One solution to this problem is to measure the back EMF of the various pumps in the series during manufacture and apply a calibration factor to each pump compensate for the variability. However, this does not offer a complete solution. The voltage sensor used to measure the back EMF must be capable of measuring the greatest back EMF generated by the pump with the strongest magnetic interaction, and thus must have a large dynamic range. Such a sensor may not give precise measurements when measuring a smaller back EMF generated by a pump with a weaker magnetic interaction.